Fallback option in case of missing images

[davidmccabe]

This adds a setting called fallback that is helpful when programmatically generating the name of the image to load, in case that the image might not exist. For instance, I am developing a site where most of the pages each have an individual background image. But for those pages that are missing their own image, I want to fall back to a generic image.

Here's an example from the my site:

$.anystretch("/images/background-"+slug+".jpg",
  {fallback: "/images/background-default.jpg", speed: 300});

This will load images according to the page's slug, defaulting to the fallback image if the first one doesn't exist.

It's worth considering making the fallback a callback function rather than just another image URL, as well.